Steam generators of liquid-metal cooled fast-neutron nuclear reactors are known which comprise a bundle formed by an assembly of parallel straight tubes fixed at their ends in tube plates parallel to one another and fastened to the outer casing of the steam generator. In the interior of the steam-generator casing the tube plates delimit three successive zones, namely, a water container, a circulation zone for the heat-transporting liquid metal in contact with the outer surface of the tubes of the bundle, and a steam header.
Through their entire thickness the tube plates are pierced by holes, in the axis of each of which the end of a tube is fixed by welding to a tubular connector projecting from the inner face of the tube plate, this connector generally being referred to as a "nipple".
Each of the tubes of the heat exchanger bundle is fixed at one end on a first connector of a first tube plate, and at its other end on a second connector of a second tube plate, the inside diameter of the second connector and of the corresponding opening of the second tube plate being slightly greater than the outside diameter of the tube which is connected to the second connector by a junction zone of frustoconical shape.
Each of the tubes thus forms the connection between the water container and the steam header, the feed water of the steam generator, distributed over all the tubes of the bundle at the outlet of the water container, then being heated and vaporized inside the tubes through thermal contact with the heat-transporting liquid metal. The steam formed in the tubes is then recovered in the steam header.
The hot liquid sodium is generally brought to the top part of the steam generator, in the vicinity of the tube plate delimiting the steam header; the liquid sodium then circulates vertically outside the tubes of the bundle.
If one of the tubes is defective (for example, cracked) so that it leaks, this leak must be detected very quickly, so that the operation of the steam generator can be halted and the generator emptied before the reaction between the liquid sodium and the water flowing through the leak progresses to such an extent that the steam generator may be damaged and it will no longer be possible to master this reaction with the safety means provided on the steam generator, which damp the pressure waves in the sodium.
A leak in one of the steam generator tubes can be discovered either by detection of the hydrogen which is formed by the reaction of the water on the sodium, or by acoustic detection of the noise of the reaction.
After complete emptying of the steam generator, it is necessary to carry out a repair on the leaking defective tube, either by plugging or by replacement in order to enable the steam generator to be put back into service.
For the plugging of a defective steam-generator tube various techniques have been proposed, as described for example in FR-A-2,524,609 and FR-A-2,560,962.
In FR-A-2,524,609, it is proposed to effect the plugging of the tube by automatic displacement of a valve situated inside the tube, in such a manner as to close off the tube through the action of the difference in pressure accompanying the occurrence of the leak and the consequent reaction between the sodium and the water. This plugging of the tube makes it possible to prevent the leak from having harmful consequences, by reliably and immediately separating the water or steam under pressure from the sodium.
In FR-A-2,560,962 it is recommended to effect the plugging by placing in position a flexible sleeve, disposed axially inside the tube and fixed by mechanical keying or by explosive welding.
Actually, plugging techniques making use of expanding sleeves, valves, or the fastening of a closure means by flanging, explosive welding or keying, are no longer in use within the scope of maintenance or repair operations for steam generators utilizing liquid sodium as primary fluid. It is in fact necessary during operation to maintain absolute leaktightness between the water and the liquid sodium. None of the techniques mentioned above makes it possible to ensure such absolute leaktightness and its maintenance over a period of time.
In addition, the proportion of tubes closed off in the course of successive repair operations on a steam generator must not exceed a certain percentage of the total number of tubes.
It is generally considered that this percentage should be between 10 and 15% of the tubes.
The technique of plugging tubes has the disadvantage of entailing a loss of power of the steam generator and of introducing temperature asymmetry in the bundle of tubes, so that the other tubes of the bundle are subjected to increased thermomechanical stresses.
Furthermore, for reasons of safety it is not possible to place plugs on the outer walls of the tube plates of a steam generator, i.e., on the water-container side or on the steam-header side. In this case, in fact, the detection of hydrogen or acoustic detection enabling the occurrence of a water or steam leak into the sodium to be discovered cannot be achieved reliably and quickly. Belated detection of water leakage into the sodium entails very serious disadvantages, which have been mentioned above.
Tube-closing plugs must therefore be placed on the inside walls of the tube plates, i.e., the walls directed towards the zone of the steam generator enclosing the sodium and the bundle of tubes. Tube plates have thicknesses which may be relatively considerable, for example of the order of 400 mm, so that the plugs must be placed on walls situated at substantial distances from the outside access faces of the tube plates. This makes it difficult to place the plugs in position.
Moreover, the welding of a plug closing a tube in a steam generator in which the primary fluid consists of sodium is just as delicate an operation as the welding of the original tube which has to be plugged, and must be carefully checked.